There are many web-based color patterning processes in use today. Such systems typically include a continuous web of manufactured material (e.g. paper, textiles, linoleum) that passes through a printing press which imparts colored designs and patterns to the material using various dyes or inks. These designs are typically manufactured as a series of adjacent frames on the material, with each frame containing the printed pattern. One common printing press for such processes utilizes a plurality of cylindrical screens that rotate in contact with the material. Ink or dye is squeezed through the screen onto the material at predetermined times and locations to apply a pattern to the material that is repeated with each rotation of the screen. Ink jet printers that directly spray the colors onto the moving web may also be used.
Three categories of defects may occur during this printing process. First, "the right color in the wrong place" means the color is applied at incorrect locations on the material. This problem may be global and occur over the entire surface of material, or it may be local to a portion of each frame. Second, "the wrong color in the right place" means there is a problem with the dye delivery. The color may have insufficient coverage (scabby), be of an incorrect formulation (off shade), be applied too thickly (mottled), or be not printed (missing). These defects are frequently global, although stickins (a miscellaneous splotch of color) are local. Third, material defects, including seam marks, scrimps, print resist, butterflies, glue streaks, crooked cloth, and bias printing, also result in global or local defects.
Recently, interest has been growing in the use of computer-based inspection systems to monitor, in real time, the print process to provide timely information about the overall print quality, the presence of print defects that would require immediate remedial action, and an electronic "roll-map" of an entire batch of printed material. Such systems could significantly enhance the value added in a color print process by improving quality control, reducing waste, and allowing automated inventory management.
The technical requirements of such a system are ambitious. Many color patterning processes utilize very high web speeds and widths. The spatial detail of printed patterns demand very high resolution imaging systems. These two requirements imply very high raw sensor data rates. Camera and computer system capabilities are just recently achieving the level of performance to permit such realtime inspection.
U.S. Pat. No. 5,301,129 of S. McKaughan et al discloses a system for inspecting the surface of a moving web of homogeneous material using a camera to receive light reflected from the web. A series of multi-dimensional convolution filters transform the output of the camera to eliminate background information while enhancing the features of anomalies. The patent does not disclose application of this system to a patterned web.
U.S. Pat. No. 5,533,145 of F. Shofner et al discloses a system for detecting fiber-size defects in textile webs using light reflected from the web and computer processing. This patent does not disclose web pattern detection.
Statutory Invention Registration No. H1616 of K. Wolfe, dated Dec. 3, 1996, discloses a web inspection system having enhanced video preprocessing of an image captured by a line scan CCD camera. This system does not disclose comparing successive frames of a web.
U.S. Pat. No. 5,696,591 of R. Bilborn et al discloses a system that measures light intensity transmitted through a web and triggers a fault when the received intensity differs from a profile. The image is digitized and processed in real time.